Pilus
proteinaceous hair-like appendage on the surface of bacteria ranging from 2-8 nm in diameter
A pilus (Latin for 'hair'; pl.: pili) is a hair-like cell-surface appendage found on many bacteria and archaea. The terms pilus and fimbria (Latin for 'fringe'; plural: fimbriae) can be used interchangeably, although some researchers reserve the term pilus for the appendage required for bacterial conjugation. All conjugative pili are primarily composed of pilin – fibrous proteins, which are oligomeric.
Dozens of these structures can exist on the bacterial and archaeal surface. Some bacteriophages (bacterial viruses) attach to receptors on bacterial pili at the start of their reproductive cycle.
Pili are antigenic. They are also fragile and constantly replaced, sometimes with pili of different composition, resulting in altered antigenicity. Specific host responses to old pili structures are not effective on the new structure. Recombination between genes of some (but not all) pili code for variable (V) and constant (C) regions of the pili (similar to immunoglobulin diversity). As the primary antigenic determinants, virulence factors and impunity factors on the cell surface of a number of species of gram-negative and some gram-positive bacteria, including Enterobacteriaceae, Pseudomonadaceae, and Neisseriaceae, there has been much interest in the study of pili as an organelle of adhesion and as a vaccine component.
The public source identifies “Pilus” as proteinaceous hair-like appendage on the surface of bacteria ranging from 2-8 nm in diameter. This brief keeps that definition visible, then builds a research path around Pilus, proteinaceous and hair-like.
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Named sources, stable identifiers and responsible institutions provide the strongest route from overview to verifiable evidence. The source revision retrieved here is dated Sep 9, 2026. The linked authority identifier is Q934786. None of the 0 selected statements returned an explicit reference.
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This entry incorporates text from “Pilus” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.